The subject matter disclosed herein relates to turbine airfoils, and more particularly to airfoil heat shields.
Airfoils (i.e., vanes and blades) are typically disposed in hot gas paths of gas turbines. A blade, which can also be referred to as a “bucket” or “rotor”, can include an airfoil mounted to a wheel, disk or rotor, for rotation about a shaft. A vane, which can be referred to as a “nozzle” or “stator”, can include an airfoil mounted in a casing surrounding or covering the shaft about which the blade rotates. Typically, a series of blades are mounted about the wheel at a particular location along the shaft. A series of vanes can be mounted upstream (relative to a general flow direction) of the series of blades, such as for improving efficiency of a gas flow. Vanes succeeded by blades are referred to as a stage of the gas turbine. Stages in a compressor compress gas, for example, to be mixed and ignited with fuel, to be delivered to an inlet of the gas turbine. The gas turbine can include stages in order to extract work from the ignited gas and fuel. The addition of the fuel to the compressed gas may involve a contribution of energy to the combustive reaction. The product of this combustive reaction then flows through the gas turbine. In order to withstand high temperatures produced by combustion, the airfoils in the turbine need to be cooled. Insufficient cooling results in undue stress on the airfoil and over time this stress leads or contributes to fatigue and failure of the airfoil. To prevent failure of turbine blades in gas turbine engines resulting from operating temperatures, film cooling has been incorporated into blade designs. In film cooling, cool air is bled from the compressor stage, ducted to the internal chambers of the turbine blades, and discharged through small holes in the blade walls. This air provides a thin, cool, insulating blanket along the external surface of the turbine blade. Film cooling can be inefficient because it can create non-uniform cooling, since close to the holes the film temperature is much cooler that farther away from the holes. Accordingly, a need exists for improved cooling of the airfoil.